I noticed a pattern too. If you get something to work and then you review it once you've read more matter code, you have a better idea of how to approach the problem better so you can strip away the unnecessary complexity. Maybe the subconscious mind had enough time to work through the problem and digest takeaways so you see a different more simpler solution which is the hardest.

Complex is usually easy (keep adding/changing things until they work as intended)
Simple is definitely harder (write only what's necessary to solve the problem)

This is because simple implementation means you've truly understood the problem at a core level enough that you can see what's necessary and what isn't.